This invention generally relates to an emulsion polymerized latex copolymer used as a binder in paper-coating compositions. More particularly, the invention relates to paper-coating compositions containing such latex and to papers coated with such paper-coating compositions.
Colloidally stable aqueous dispersions of polymers, which dispersions are typically referred to in the art as latexes, are generally known to be useful alone and in various formulations as coatings and impregnants for various substrates. A wide variety of such latexes of differing homopolymeric and copolymeric compositions have been developed with specific chemical and/or mechanical properties for particular end-use applications. Specifically, many latexes have been developed for use in paper-coating compositions. Paper coating compositions are generally combinations of a binder (latex and/or "natural" binder) and a pigment. Paper-coating compositions are applied to the surface of paper in order to give the paper greater stiffness, opacity, whiteness, brightness, gloss, smoothness and/or ink receptivity.
Latexes employing a monovinylidene aromatic monomer such as styrene and an aliphatic conjugated diene such as butadiene have been very popular for the production of latexes, especially those designed for paper-coating compositions. In these compositions, butadiene acts somewhat as a cross-linking agent, giving the latexes excellent properties. Recently, attempts have been made to produce paper-coating latexes which do not include significant amounts of butadiene. Specifically, attempts have been made to produce latexes from acrylic esters. Since the monomers in these formulations do not naturally cross-link the resulting polymer latexes, it is necessary to introduce a separate cross-linking agent such as divinyl benzene. While the performance of these latexes (with cross-linking agent) might be expected to be equivalent with styrene/butadiene latexes, they are in fact considerably inferior in performance. Specifically, these latexes suffer from poor dry and wet binding strength.
Since price projections for these acrylic monomers are favorable, and since their polymers have better light stability and age stability than styrene/butadiene polymers, it would be desirable to have an acrylic latex which does not contain an aliphatic conjugated diene, but which has performance in paper-coating compositions equal to formulations containing an aliphatic conjugated diene.